Date: 31/10/12
Page 10File: 700b Technical Manual ver1.9.3.docm
3.5 Analogue
Configuration
3.5.1 Sensor
Calibration
Sensors are calibrated by providing two sets of coordinates (4 values) from a graph,
from these coordinates the
slope
of the graph can be calculated, this is also called the
multiplier (M)
.
Reading in Engineering units (Y) = multiplier (M) * sensor reading in mA/Volts (X) +
offset (C )
By making fine adjustments of the set points and using an external calibration reference
we can calibrate the sensor.
3.5.2 Calibration
Example
1
You may have trouble doing fine adjustment of slope, by adjusting the high or low
calibration points. The EDAC700 RTU may give you the impression that it does not use
the decimal component of the value entered, making it feel like it has quite coarse
control over calibration values. The EDAC700 RTU actually uses 5 decimal places more
than the decimal places entered in the calibration values. Therefore entering 3.12 will
cause the EDAC700 RTU to use a value of 3.1200000 and entering 3.1234 will mean
3.123400000 is used.
The formula for calibration of slope is:
mA x steps x multiplier = display value (assume offset =0) & V x steps x multi = display
value
where steps 1V = 4095, mA = 4095/20
eg at 1:1 becomes: 20mA x 4095/20 x 0.004884 = 20
To calibrate the multiplier:
1. Set up reading as desired with low/high mA and Eng units
2. Read the display. (D1) eg 19.5
3. Note the expected display (D2) eg 20.0
4. Calculate correction factor D2/D1 (20.0/19.5 = 1.025)
5. Adjust mA high value by correction factor HmA x D2/D1 (20 x 1.025 = 20.51)
6. The display should now read as expected.
C=Offset
Y=Eng Units
Hi Eng
= 100%
Low Eng
= 0%
Y = M * X + C
X = mA
Low Value
= 4mA
High Value
= 20mA
